The preparation of polyurethanes and polyurethane/urea elastomers by reacting an aromatic isocyanate with a polyol and then chain extending with a short chain diol or aromatic diamine to form the elastomer is well known. Two processes are used, namely, the prepolymer process and the one shot process which includes reaction injection molding (RIM). A reactant system widely used in the prepolymer process utilizes toluenediisocyanate as the isocyanate component of the prepolymer and polytetramethylene glycol as the polyol component. (Such a system is sold under the trademark Adiprene). Then, the prepolymer is contacted with an aromatic diamine chain extender and the formulation is molded.
The prepolymers using toluenediisocyanate as the isocyanate component have suffered because toluene diisocyanate is volatile and toxic thus requiring special handling procedures. Another is that free toluenediisocyanate remains in the prepolymer and unless extracted, is too reactive for use with all but inactive chain extenders, e.g., methylene bis(orthochloroaniline) (MOCA). U.S. Pat. Nos. 2,969,386; 3,285,951 and 3,218,348 disclose some of the problems associated with the use of toluenediisocyanate for use in urethane systems.
The prior art has produced a variety of prepolymer systems from toluenediisocyanate (TDI) and long chain polyols and such prior art then include the following patents:
McGillvary, U.S. Pat. No. 3,701,374, discloses a polyurethane pneumatic tire with excellent strength, elasticity, tear resistance, flat-spotting properties, and overall performance in service, both at ambient and elevated temperature. The elastomer utilized is obtained by chain extending a polyisocyanate prepolymer, prepared by reacting a stoichiometric excess of toluenediisocyanate with 1000 molecular weight (mw) poly(tetramethylene glycol) at 70.degree.-80.degree. C. PA0 Kaneko, et al, U.S. Pat. No. 3,963,681, discloses urethane elastomers having improved mechanical properties. The prepolymer is prepared by reacting a stoichiometric excess of TDI with a polyether polyol having an average mw range of 1000 to less than 450 and a molecular weight distribution curve with at least 2 peaks, one at higher and one at lower mw range. The prepolymer then is chain extended with a curing agent. PA0 Schroeder, U.S. Pat. No. 4,029,730, discloses a cordless vehicle tire with an elastomeric polyurethane sidewall from a polyisocyanate prepolymer chain extended with a methylenedianiline/salt complex. The polyisocyanate prepolymer utilized is prepared by reacting a stoichiometric amount of toluenediisocyanate with a polyol at 80.degree.-90.degree. C. for 2-4 hrs. PA0 Vial, et al, U.S. Pat. No. 4,089,822, discloses a polyurethane elastomer exhibiting a low glass transition temperature. It is formed by reacting a polymeric diol with a slight stoichiometric excess of toluenediisocyanate, and chain extending the resultant prepolymer with 2,6-dichloro-p-phenylene-diamine. PA0 Blahak, et al, U.S. Pat. No. 4,133,943, describe an improved process for the production of polyurethane/urea elastomers, in which diamines containing ester groups are used as chain extenders. The polyisocyanate prepolymer utilized in the system was prepared from a polyol and a slight stoichiometric excess of toluenediisocyanate. PA0 Chung, et al, U.S. Pat. No. 4,365,051, describe polyurethanes prepared from diamines, substituted butylbenzoate, benzonitriles, or benzoates, and polyisocyanate prepolymers. The prepolymers are prepared by reacting a stoichiometric excess of aromatic diisocyanate with polyol. PA0 1. there is unreacted toluenediisocyanate in the prepolymer and its presence may cause toxicity and reactivity problems, PA0 2. stoichiometric excesses of toluenediisocyanate may cause high oligomer content leading to non-uniformity in elastomer hard segments, and PA0 3. high temperature prepolymer processing results in by-product formation, probably biuret and trimer by-product, which allows hard segment crosslinking and therefore results in lower isocyanate content. PA0 an ability to use commercial isomer mixtures of toluenediisocyanate to produce the prepolymer; PA0 a prepolymer system capable of producing elastomers having outstanding high temperature physical properties including low hysteresis; PA0 a prepolymer which can be used for producing an elastomer having excellent thermal properties; and PA0 a relatively straightforward process utilizing conventional processing equipment and techniques.
Although the elastomers above have desirable properties, the above mentioned prepolymer preparations have shortcomings in terms of many commercial applications, they are:
There is some elastomer art which discloses the use of prepolymers free of unreacted toluenediisocyanate. This art is as follows:
Jackle, U.S. Pat. No. 4,182,825, shows polyurethanes with superior dynamic properties produced from polyisocyanate prepolymers, free of unreacted toluenediisocyanate (TDI). The TDI is removed by thin film distillation. A polyisocyanate prepolymer is prepared by reacting a polyether polyol with a stoichiometric amount, e.g., 1.97:1 NCO to OH or slight excess of 80/20 2,4-/2,6-TDI and then removing the unreacted TDI.
Megna, U.S. Pat. No. 4,556,703 discloses the preparation of polyurethanes using toluenediisocyanate feed systems high in 2,6-isomer content for the preparation of the prepolymer. After prepolymer formation, the excess or unreacted toluenediisocyanate is removed. The higher concentrations of 2,6-isomer result in elastomers having lower heat buildup on flexing.
Schmidt, et al, U.S. Pat. Nos. 4,507,459 & 4,519,432, describe the synthesis of polyurethanes having low hysteresis. A prepolymer of a mononuclear aromatic diisocyanate and polyol are reacted with a chain extender mixture of mononuclear aromatic diamine and polyol. The polyisocyanate prepolymer is prepared by reacting a stoichiometric excess of toluenediisocyanate with polyol.
Some patents although not relevant for polyurethane synthesis show techniques for separating the 2,4-toluenediisocyanate isomer from the 2,6-isomer by reaction with a long chain polyol. For example, U.S. Pat. No. 3,554,872 shows reacting toluenediisocyanate with a long chain diol at a mole ratio of about 3.5 to 1 toluenediisocyanate to diol at 25.degree.-30.degree. C. Distillation of the prepolymer to remove unreacted toluenediisocyante was carried out at temperatures from 130.degree.-150.degree. C.